Here's the program.  It's nothing too great, but it should work.  Just run the
program and select all the HyperCard.bxy (they MUST end in .BXY, or else you
can't see them) files you want changed (you can select them all in one go) and
the program will remove the 512 byte header from them.  It will rename the files
you select as "<file>.OLD" and rewrite the changed ones as "<file>.BXY" so if it
screws up, you still have the originals.  The dialog will also show all files
ending in .OLD, so you won't accidentally do it twice.  You will need enough disk
space for 2 copies of each file.  Once it's done, you should be able to unpack
the new .BXY files normally.  The dialog will also show all files ending in .OLD,
so you won't accidentally do it twice.

There isn't much error handling to speak of; you'll just get a message with a
couple of numbers.  The most likely error will be not enough disk space, though. 
If you have any problems, just let me know.

Good luck!
Mark Marr-Lyon
o9938156@unicorn.it.wsu.edu